1. Field of the Invention
The present invention relates generally to wireless communication systems. More particularly, the present invention relates to a frequency synthesis system having a voltage controlled oscillator and resonant circuit that generates the local oscillator frequencies required by a wireless communication system which must rapidly change between two different frequencies, such as a transmit frequency and a receive frequency.
2. Description of the Related Art
As with most electronics devices, the increasing popularity of wireless communication systems is forcing the developers of these systems to continually innovate in order to compete in the industry. Such areas of innovation include the minimization of the size of the wireless transceivers and the decrease of power consumption of the device in order to provide for extended periods of battery-powered operation.
One method of minimizing the size of the wireless transceiver is to develop multi-purpose semiconductor components that serve more than one function within the transceiver. Despite the development of these multi-purpose semiconductors, the frequency synthesis devices currently available on the market typically require two different resonant circuits in order to generate different local oscillator (LO) frequencies, such as the receive LO frequency and the transmit LO frequency. This redundancy may include having two (2) separate voltage controlled oscillators (VCO) each having its own resonant circuit for generating the two LO required to operate the transceiver. Unfortunately, the redundant VCO and resonant circuit requires a great deal of power, even if each VCO is only powered during its respective transmit or receive period.
One method of overcoming the redundancy of a two VCO frequency synthesis system is the development of a VCO having two different resonant circuits. In this solution, each resonant circuit is tuned to generate its particular LO frequency, and the VCO is switched between the two resonant circuits in order to generate the two LO frequencies. Although this solution eliminates one VCO from the transceiver, the time period required for the switching between resonant circuits and the necessary settling time for that circuit, prohibits the rapid switching between the two LO. Moreover, the incorporation of two resonant circuits, and the isolation needed due to their susceptibility to electromagnetic interference (EMI), results in a VCO circuit of considerable size.
The inability of the frequency synthesis system to rapidly switch from one frequency to another is further apparent when attempting to change from a LO frequency of 600 MHZ to 900 MHZ, for example. Such a frequency change is required for many wireless transceiver applications because the transmit frequency band is often separated from the receive frequency band by a large isolation band of several hundred megahertz. While this frequency difference provides for a full duplex wireless communication system having lower channel interference, the number of components of various sizes needed to create such a wide band resonant circuit is high.
In light of the above, it would be advantageous to provide a frequency synthesis system which minimizes both the size of the required circuitry, the number of active and discrete components, and the power required to operate the system. Further, it would be advantageous to provide a frequency synthesis system having a VCO circuit that would incorporate only a single resonant circuit that may be quickly modified to facilitate the generation of two LO frequencies using a single VCO.